Zircon U–Pb and Lu– Hf isotope evidence from the Kimberley and Speewah groups, Northern Australia
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Changes in the provenance of sedimentary successions through time can provide valuable information on the nature and timing of tectonic processes related to basin formation and on possible sediment source regions. The Paleoproterozoic Speewah Group and unconformably overlying Kimberley Group of northern Western Australia are sandstone-dominated fluvial and shallow-marine sedimentary successions that also contain siltstone, claystone, mafic volcanic rocks, and rare carbonate rocks. Both groups are intruded by sills and dykes of the c. 1797 Ma tholeiitic Hart Dolerite, which constitutes an important time marker. Deposition of the Speewah and Kimberley Groups occurred during and immediately following the 1835–1810 Ma Halls Creek Orogeny, which resulted from collision of the North Australian Craton and Kimberley Craton. However, the tectonic drivers for basin formation and associated mafic volcanism are still unclear. To understand these drivers, we present new SHRIMP U–Pb age and LA-MC-ICPMS Lu–Hf isotope data for detrital zircons from the Speewah and Kimberley Groups. The results reveal a marked change in provenance at the base of the Kimberley Group. The Speewah Group is dominated by Paleoproterozoic (1880–1850 Ma) detrital zircons with unradiogenic Hf (Hf = –8 to +1), whereas the onset of deposition of the Kimberley Group is marked by a dramatic increase in the proportion of mainly Neoarchean (2525–2480 Ma) detrital zircons with radiogenic Hf (Hf = +0.5 to +7).We propose that the Speewah Group was derived largely from erosion of 1865–1850 Ma Paperbark Supersuite granites during the Halls Creek Orogeny. We also propose that the increase in Neoarchean detritus in the Kimberley Group reflects post-orogenic extensional collapse at c. 1800 Ma that resulted in uplift and erosion of currently unexposed Neoarchean basement of the Kimberley Craton. A Neoarchean source from within (rather than distal to) the Kimberley Craton is also supported by the isotopic character of the Paperbark Supersuite, which is consistent with magmatic reworking of a juvenile Neoarchean source. Paleocurrent directions in the Kimberley Group, correlation of age and isotopic characteristics of potential source rocks, and a recent paleomagnetic tectonic reconstruction are consistent with a possible sediment source region in the Dharwar Craton, which may have been contiguous with the Kimberley Craton at c. 1800 Ma.
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